Computers & Laptops

PSU Wattage Calculator

The power supply capacity a system needs, from its load and the headroom you want above it.

5 inputs Free, no sign-up

Your figures

From the manufacturer's specification for the exact part you have, or measured with a plug-in energy meter. This page holds no hardware data - a figure invented for "a graphics card" would be wrong for most of them.

Total board power for your exact model. Leave blank if there is no separate card.

Motherboard, memory, drives, fans and cooling added together.

If you already know the total, enter it here instead of the three fields above. Anything you fill in is added, so do not enter both.

Spare capacity above the load. This is a choice rather than a rule, and the result names whichever figure it used.

Try an example

Result

Your result

Enter your figures and the result appears here.

Estimates only. Rates, fees and specifications change. Confirm against the official source before you rely on a figure — see our disclaimer.

About this calculator

A power supply should be able to carry the load with room to spare. How much room is a judgement, and this page makes it an input rather than pretending there is a rule.

Enter the processor, the graphics card and everything else — or one total if you have it — choose your headroom, and the result is the capacity to look for.

How to use this calculator

  1. Enter the processor's rated power from its specification.
  2. Enter the graphics card's total board power, or leave it blank if there is no separate card.
  3. Add everything else as one figure — board, memory, drives, fans, cooling.
  4. Set the headroom you want. Thirty per cent is this page's starting figure.

Not sure of the total? The PC power calculator works it out component by component.

The formula

Total load        = CPU + GPU + everything else
Required capacity = Total load x (1 + headroom / 100)

The result also shows what that capacity would be running at — a 30% headroom means the supply sits at about 77% of its rating under full load, which is the figure the headroom convention is really about.

SymbolMeaningUnit
Total load Everything the supply carries W
Headroom Spare capacity you want %

Worked example

A 120 W processor, a 250 W card and 100 W of everything else, at 20% headroom:

Total     = 120 + 250 + 100 = 470 W
Required  = 470 x 1.20       = 564 W

So a 600 W supply, running at about 78% of its rating when everything is working hard.

Notes and limits

  • The advice to size a supply at twice the load is a convention, not a specification. It comes from efficiency curves peaking near half load and from leaving room to upgrade.
  • A supply running near its rating is not faulty. It is hotter, louder and closer to its limit, which is a different thing from unsafe.
  • Buying far more capacity than you need costs money and gives up a little efficiency, because supplies are least efficient at very low load.
  • This gives a wattage. Which unit to buy also depends on its connectors, its size, its protections and its behaviour under transient load, and none of those is a number.
  • To check a supply you already own, use the PSU load calculator.

What this calculation assumes

  • Every wattage is one you entered. No component or supply data is stored in this page.
  • The headroom is your choice and the result names the figure it used.
  • Transient spikes above the rated figures are not modelled.

Frequently asked questions

What size power supply do I need?

Add up what your components draw and multiply by one plus the headroom you want. A 470 W system with 20% headroom points at 564 W, so a 600 W unit. The headroom is your choice, not a rule.

Is a bigger power supply always better?

No. Beyond enough headroom for the load and a future upgrade, extra capacity costs more and buys nothing - and a supply running at very low load is slightly less efficient than one in the middle of its range.

Should I use the graphics card maker's recommended wattage?

As a cross-check. That figure is for a whole system with headroom already included, so it should be compared against this result rather than added to the component total.

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